A177997 p and A002808(p)/2 are both prime.

2, 5, 7, 13, 31, 41, 43, 59, 101, 107, 127, 137, 149, 199, 239, 277, 359, 389, 479, 613, 743, 757, 809, 829, 937, 991, 1031, 1103, 1439, 1487, 1499, 1847, 1877, 2011, 2083, 2179, 2609, 2663, 2711, 2741, 2749, 2857, 2909, 3329, 3559, 3623, 3643, 3697, 3823

Offset: 1

Author

Juri-Stepan Gerasimov, May 17 2010, May 23 2010

Keywords

• nonn

Comments

Primes p such that composite(p) is an even semiprime.

Examples

			a(1)=2 because 2=prime and composite(2)/2=6/2=3=prime.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002808, A065858, A065897.

Programs

Maple

P,C:= selectremove(isprime, [$2..10000]):
select(t -> t <= nops(C) and C[t]::even and isprime(C[t]/2), P); # Robert Israel, Mar 20 2018

PARI

c=[]; for(n=2, 10000, if(!isprime(n), c=concat(c, n))); c; \\ The composites
s=[]; forprime(p=2, #c, if(c[p]%2==0 && isprime(c[p]\2), s=concat(s, p))); s \\ Colin Barker, Jun 28 2014

Extensions

Corrected by D. S. McNeil and R. J. Mathar, May 23 2010